This invention relates to an optical time division demultiplexing apparatus and a demultiplexed signal switching method as well as an optical time division multiplex transmission system suitable for use with an optical communication system which adopts an optical time division multiplex transmission method.
In recent years, as increase in information amount proceeds suddenly, increase in capacity of an optical communication system is demanded. At present, while an optical amplification multiplex repeating system having a transmission rate of 10 Gb/s (gigabit/second) is about to be put into practical use, since further increase in capacity is estimated for such a repeating system as just mentioned, development of an optical communication system which has a higher transmission rate is required
In conventional large capacity optical communication systems, it is a principal countermeasure to increase the speed of operation of an electronic circuit for both of a transmission section and a reception section in order to achieve increase in capacity. In recent years, however, increase in speed of operation of an electronic circuit has been and is getting difficult. For example, also with electric devices for optical communication using Si, GaAs, HBT, HEMT and so forth to which investigation and development are directed at present, it is said that the transmission rate which can be put into practical use at present is 10 Gb/s to the utmost.
Therefore, in order to achieve increase in transmission rate of an optical transmission system to a rate higher than the working speed of an electronic device, it is considered effective to utilize multiplexing-demultiplexing in an optical region. In particular, methods of performing multiplexing-demultiplexing on the optical wavelength base or methods of performing bit multiplexing-demultiplexing on the time base seem available, and in recent years, investigation for and development of multiplexing-demultiplexing techniques which employ such methods as mentioned above have been and are being performed energetically by various institutes.
FIG. 32 is a block diagram showing an optical transmission-reception system (optical time division multiplex transmission system) which adopts an optical time division multiplexing and demultiplexing method (OTDM) described above wherein bit multiplexing-demultiplexing is performed on the time base. Referring to FIG. 32, the optical transmission-reception system is generally denoted at 100 and performs bit multiplexing-demultiplexing of an optical time division multiplex signal which consists of two signal series (hereinafter referred simply as series) of light signals. It is to be noted that also an optical transmission-reception system for three or more series can be formed similarly to the optical transmission-reception system shown in FIG. 32.
In the optical transmission-reception system 100 shown in FIG. 32, an optical transmitter 102 and an optical receiver 103 are connected to each other by a transmission line 101 formed from an optical fiber or a like element.
The optical transmitter 102 includes two optical modulation elements 104 and 105 and an optical time division multiplexing element 106 and is constructed such that a light signal including data signal information (B gigabit/second, for example, 20 Gb/s) modulated independently of each other by the two optical modulation elements 104 and 105 is optically time-division multiplexed by the optical time division multiplexing element 106 and outputs a resulting signal as an optical time-division multiplex signal (OTDM signal) of 2.times.B gigabit/second (for example, 40 Gb/s), which is transmitted by the transmission line 101.
The optical transmitter 102 described above includes, for example, as shown in FIG. 33, a laser diode 106a, a modulation element 106b, an oscillator 106c, a branching element 106d, a delay element 106e, modulation elements 106f and 106g and a combining element 106h.
The laser diode 106a emits CW light (Carrier Wave; light of a fixed level) as pump light. The modulation element 106b modulates the pump light from the laser diode 106a with a clock signal having a frequency of 20 GHz from the oscillator 106c.
The branching element 106d branches the light signal, which has been modulated by a clock signal component of 20 GHz by the modulation element 106b, into two. The delay element 106e delays one of the two signals branched by the branching element 106d, for example, by a half period of the clock signal of 20 GHz.
The modulation elements 106f and 106g modulate clock light pulses (data having a transmission rate of, for example, approximately 20 Gb/s) modulated by the optical modulation element 104 described above and outputs a resulting signal as an RZ (Return-to-Zero) light signal.
For example, the modulation element 106f modulates the clock light pulse signal using an electric input signal of 20 Gb/s at a timing at which a clock pulse of the light signal from the delay element 106e rises, and the modulation element 106g modulates the clock light pulse signal using another electric input signal of 20 Gb/s independent of the former electric input signal of 20 Gb/s at another timing at which a clock pulse of the light signal itself branched by the branching element 106d rises.
The combining element 106h combines the light signals (data of 20 Gb/s) modulated at different timings from each other by the modulation elements 106f and 106g and outputs a resulting light signal at a transmission rate of 2.times.20 Gb/s.
Meanwhile, the optical receiver 103 of the optical transmission-reception system 100 shown in FIG. 32 includes a light branching element 107, optical demultiplexers (DEMUX) 108 and 109, and identification elements 110 and 111.
The light branching element 107 receives an optical time division multiplex signal (2.times.B gigabit/sec, for example, 40 Gb/s) from the optical transmitter 102 via the transmission line 101 and power branches the received light signal into two. The two branched optical time division multiplex signals are outputted to the two optical demultiplexers 108 and 109.
The optical demultiplexers 108 and 109 modulate the optical time division multiplex signal branched by the light branching element 107 at different timings from each other to demultiplex the optical time division multiplex signal into signals modulated by the optical modulation elements 104 and 105 described above and outputs the demodulated signals.
Each of the optical demultiplexers 108 and 109 described above can be formed, for example, as shown in FIG. 34, from a Mach-Zehnder optical switch 112 of the 1-input 1-output (1.times.1) type and a driving circuit 113 for supplying driving voltages to the optical switch 112. However, for the optical switch 112, a 1-input 1-output field absorption optical switch may be used in place of a Mach-Zehnder optical switch.
The optical switch 112 has a waveguide formed such that it is first branched into two and then combined back into one. The optical switch 112 further has a pair of electrodes 112a and 112b for individually applying driving voltages to the two divided waveguide portions. Consequently, a light signal inputted to the optical switch 112 is branched on the waveguide and then passes the waveguide portions which have electric fields formed by driving voltages which will be hereinafter described, and is then combined by the combining portion and then outputted.
The driving circuit 113 receives a clock signal corresponding to the frequency of a clock signal generated by the oscillator 106c of the optical time division multiplexing element 106 on the transmission side and supplies mutually inverted (complementary) driving voltages relative to each other to the electrodes 112a and 112b.
In particular, as shown in FIG. 35, the driving circuit 113 supplies mutually inverted (complementary) driving voltages b and c relative to each other to the electrodes 112a and 112b so that one of two signals which form an inputted optical time division multiplex signal a, that is, a demultiplexed light signal d, is outputted from the optical switch 112.
It is to be noted that, if the driving voltage c is used as the driving voltage to be supplied to the electrode 112a described above and the driving voltage b is used as the driving voltage to be supplied to the electrode 112b, then the other of the two signals which forms the optical time division multiplex signal a, that is, the other demultiplexed light signal d', can be outputted.
More particularly, if the driving voltage b is used as the driving voltage to be supplied to the electrode 112a of the optical switch 112 which forms the optical demultiplexer 108 and the driving voltage c is used as the driving voltage to be supplied to the electrode 112b, then from an optical time division multiplex signal a (40 Gb/s) inputted from the light branching element 107, for example, a light signal (demultiplexed light signal d) having data information of 20 Gb/s modulated by the optical modulation element 104 can be demultiplexed.
Similarly, if the driving voltage c is used as the driving voltage to be supplied to the electrode 112a of the optical switch 112 which forms the optical demultiplexer 109 and the driving voltage b is used the driving voltage to be supplied to the electrode 112b, then from an optical time division multiplex signal (40 Gb/s) from the light branching element 107, for example, a light signal (demultiplexed light signal d') having data information of 20 Gb/s modulated by the optical modulation element 105 can be demultiplexed.
While the optical time division multiplex signal a illustrated in FIG. 35 has an NRZ (Non-Return-to-Zero) waveform, the optical demultiplexers 108 and 109 perform a similar optical demultiplexing operation also for an RZ waveform including a waveform of light pulses.
Further, the identification element 110 shown in FIG. 32 identifies actual data information from a light signal demultiplexed by the optical demultiplexer 108. Similarly, the identification element 111 identifies actual data information from a light signal demultiplexed by the optical demultiplexer 109.
Consequently, the optical receiver 103 can demultiplex an optical time division multiplex signal of 2.times.B gigabit/second (for example, 40 Gb/s) received via the transmission line 101 into two different original light signals of B gigabit/second (for example, 20 Gb/s) and identify the two light signals.
It is to be noted that, for the optical transmitter which performs such optical time division multiplexing processing as described above, in place of the optical transmitter 102 shown in FIG. 33, an optical transmitter which adopts a technique of multiplexing a light pulse signal obtained by modulating light from a short pulse light source may be used.
Or, in place of the optical receiver 103 described above which demultiplexes a duplex light signal, such an optical receiver 114 as shown in FIG. 36 may be used. In particular, the optical receiver 114 shown in FIG. 36 includes an optical demultiplexer (DEMUX) 115 and a pair of identification elements 116 and 117.
The optical demultiplexer 115 receives an optical time division multiplex signal (2.times.B gigabit/second, for example, 40 Gb/s) from the optical transmitter 102 via the transmission line 101 and performs time division demultiplexing processing for the received light signal. More particularly, as shown in FIG. 37, the optical demultiplexer 115 includes a Mach-Zehnder optical switch 118 of the 1-input 2-output (1.times.2) type and a driving circuit 119 for supplying a driving voltage to the optical switch 118.
The optical switch 118 has a waveguide formed thereon such that a light signal inputted thereto is branched into two and outputted as two light signals. The optical switch 118 further has a pair of electrodes 118a and 118b for supplying driving voltages to the two branched waveguide portions. Consequently, a light signal inputted to the optical switch 118 is branched on the waveguide, passes the waveguide portions which have electric fields provided by the driving voltages and is then outputted as two light signals.
The driving circuit 119 receives, similarly to the driving circuit 119 shown in FIG. 34 and described hereinabove, a clock signal corresponding to a frequency of a clock signal generated by the oscillator 106c of the optical time division multiplexing element 106 on the transmission side and supplies mutually inverted driving voltages relative to each other to the electrodes 118a and 118b.
Consequently, for example, as seen in FIG. 38, the driving circuit 119 supplies driving voltages b and c inverted relative to each other to the electrodes 118a and 118b so that one of two signals which form the inputted optical time division multiplex signal a, that is, the demultiplexed light signal d (for example, a light signal having data information modulated by the optical modulation element 104), is outputted from one of two output ports of the optical switch 118 and the other demultiplexed light signal e (for example, a light signal having data information modulated by the optical modulation element 105) is outputted from the other output port e of the optical switch 118.
By the way, the identification element 116 shown in FIG. 36 receives the demultiplexed light signal d from the one output port of the optical demultiplexer 115 and identifies actual data from the demultiplexed light signal d.
Similarly, the identification element 117 receives the demultiplexed light signal e from the other output port of the optical demultiplexer 115 and identifies actual data information from the demultiplexed light signal e.
Consequently, also the optical receiver 114 can demultiplex an optical time division multiplex signal of 2.times.B gigabit/second (for example, 40 Gb/s) received via the transmission line 101 back into two original light signals of B gigabit/second (for example, 20 Gb/s) and identify them.
By the way, in the Mach-Zehnder optical switches 112 and 118 which are used in the optical transmitter 102 and the optical receivers 103 and 114 described above, the powers of output lights have such a characteristic as indicated by a solid line A of FIG. 39 with respect to a potential difference between driving voltages to be applied complementarily to the two electrodes.
In this instance, as the amplitude of pulse signals to be applied from the driving circuit, such a potential difference with which the pulse signals reciprocate between "0" (a minimum output) and "1" (a maximum output) on an operation characteristic curve of the light output intensity is set. More particularly, as pulse signals to be provided by the driving voltages, such an amplitude with which the pulse signals reciprocate between Vb1 and Vb2, and also such an amplitude with which the pulse signals reciprocate between Vb2 and Vb3 can be provided.
However, while such an optical transmission-reception system 100 described above which adopts optical time division multiplexing as shown in FIG. 32 employs the Mach-Zehnder optical switch 112 for the optical demultiplexers 108 and 109 of the receiver, it has a subject to be solved in that, due to an influence of a lithium niobate construction which forms the Mach-Zehnder optical switch 112, the operating point drifts by a temperature variation or a secular change.
In particular, although the driving voltages are set such that the output intensity of the optical switch reciprocates between "0" (the minimum output) and "1" (the maximum output) of an operation characteristic curve when, for example, such an amplitude as shown in FIG. 39 that reciprocates between Vb1 and Vb2 is provided as the driving voltage, the operation characteristic curve is shifted as indicated by a broken line (B) or another broken line (C) of FIG. 39 by a temperature variation or a secular change.
In this instance, since the potential difference varies (the operating point drifts) such that it reciprocates between "0" (the minimum output) and "1" (the maximum output) on an operation characteristic curve of the light output intensity, where such an amplitude described above that reciprocates between Vb1 and Vb2 is provided, a sufficient light output intensity cannot be obtained.
Furthermore, in such an optical transmission-reception system 100 as shown in FIG. 32 which adopts optical time division multiplexing described above, the optical demultiplexer of the receiver performs optical time division demultiplexing processing for a received light signal to extract a bit train on a side required by the receiver side. However, the bit train to be extracted may be switched when necessary, and such switching must be performed efficiently taking the operating point drift described above into consideration.